Researchers at Berkeley Lab have been exploring the ways coherent synchrotron radiation (CSR) is generated in electron storage rings when femtosecond lasers are used to carve out ultrafast x-ray pulses by femtoslicing (see "Tailored Terahertz Pulses from a Laser-Modulated Electron Beam"). In their most recent work, the researchers reportedthe first observation of seeding an instability of the electron beam by the laser, and they presented a physical model that shows how this occurs under the proper conditions. Such a mechanism makes it possible to control the instability onset and to exploit its gain for the generation of pulses of terahertz CSR of unprecedented power. Terahertz radiation with a wavelength from about 1 cm to about 100 microns between the microwave and the infrared would provide access to a large number of fundamental phenomena. To mention only some of them: excited electrons orbit, small molecules rotate, proteins vibrate, superconducting energy gaps resonate, and gaseous and solid-state plasmas oscillate at terahertz frequencies. But generating terahertz radiation is ordinarily a challenging task for any kind of source, including storage-ring-based synchrotron light sources. The new findings by the ALS group could represent a significant step toward satisfying the need for powerful terahertz sources.

A Valuable Side Benefit

The ALS and other synchrotron light sources are constructed expressly to generate ultrabright beams of x rays, ultraviolet light, and even infrared for scientists to use in their experiments. But an increasingly important stretch of the vast spectrum of electromagnetic waves (or more familiarly, light) is not well served by synchrotron sources, or for that matter by any other type of light source. Known as terahertz radiation (after its frequency in the range around one trillion cycles per second), it has wavelengths bordered by microwaves on the long-wavelength side and infrared at shorter wavelengths. Moreover, it is uniquely valuable for detailed studies of orbiting electrons, rotating molecules, vibrating proteins, and many other basic phenomena. Accelerator scientists at Berkeley Lab may have a way to fill this "teraherz gap." They already are experts at the use of lasers with ultrashort pulses to carve out similarly short pulses of x rays from the longer pulses ordinarily produced at the ALS, a technique known as femtoslicing (see "Laser Time Slicing Promises Ultrafast Time Resolution"). Now they have found that extremely powerful pulses of terahertz radiation are also generated during femtoslicing. It remains to learn how to convert this phenomenon into a teraherz source useful to researchers.

In a storage ring, as in linac-based free-electron lasers (FELs), the electromagnetic field associated with the synchrotron radiation emitted by a bunch passing through an insertion device and/or a bending magnet interacts with the electrons in the same bunch, modulating their energy. Above a threshold current (number of electrons per bunch) in storage rings, the intensity becomes strong enough to exponentially amplify modulations in the bunch distribution, resulting in an effect known as the microbunching instability (MBI). Such density modulations have characteristic lengths smaller or comparable to typical terahertz radiation wavelengths and thus radiate powerful CSR pulses in that frequency range.

The FEL gain described above can amplify either small fluctuations (i.e., noise) in the electron-beam density or a small modulation in the density induced by interaction with a laser beam. The second process is referred to as laser seeding. Until now, the MBI was always observed in storage rings as a stochastic process starting from noise and associated with the emission of powerful CSR bursts with random repetition rates and large fluctuations in amplitude. But experiments performed at ALS Beamline 1.4.3 now show that by laser-inducing a modulation on the electron bunch above the MBI current threshold, it is possible to seed the instability in a controlled way. In this case, the CSR bursts become synchronous with the modulating laser, and the instability gain can be exploited for generating high-power terahertz CSR pulses.

Above the current threshold for the microbunching instability (MBI), the slicing process seeds the instability. Top: Random bursts of high-intensity, coherent synchrotron radiation (CSR) associated with the MBI, as measured at the ALS. Bottom: When the slicing laser is turned on at a 1-kHz repetition rate, the bursts become synchronous with the slicing laser, as the burst onset positions match the vertical lines at 1-ms intervals on the horizontal scale.

The bunch-current dependence of the terahertz detector's Fourier (frequency) spectra shows two prominent features: the onset of the MBI at around 15 mA and the match between the 1-kHz line and its harmonics with the laser repetition frequency. The inset shows two frequency spectra for currents above the MBI threshold with the slicing laser on (red) and off (blue). The 1-kHz line and harmonics clearly disappear when the laser is switched off.

The researchers then showed that their observations can be explained in the framework of the MBI theory. Quantitative comparison of the model with the ALS data showed good agreement up to currents of about 19 mA, thus indicating that the model can account for the CSR power growth. At higher currents, a saturation effect takes control as the MBI goes into a nonlinear regime where the theory breaks down.

Above the MBI threshold (15 mA), the total average CSR power (red) and the power associated with the 1-kHz line (green) both grow exponentially with the single-bunch current, thanks to the gain process that increases the number of electrons emitting coherently. If the power at 1 kHz were due only to the particles in the bunch that were modulated in energy by the slicing laser, then the current dependence would be quadratic, as observed below the MBI threshold. The dashed blue curves represent theoretical calculations: the exponential curve predicted by the MBI model in the top graph and a quadratic comparison in the bottom graph.

The laser-seeding phenomenon could be the basis for a THz source. Pump–probe and other experiments not requiring shot-to-shot intensity stability could benefit from the several-orders-of-magnitude increase in power that the seeded MBI offers. In a more speculative scenario, a fraction of the THz signal can be brought back into the ring to co-propagate in an insertion device or in a bending magnet with a subsequent electron bunch, modulating its energy and seeding the MBI that generates a new burst that is then used in the loop for seeding a fresh bunch. By this process, one can in principle bring the CSR emission to a stable high-power saturation regime where all the bunches radiate coherently.